Thin steel sheets and thick steel sheets with good magnetic properties have been used for magnetic shields, stators of motors, rotors, and so forth. Examples of materials with good magnetic properties include non-oriented electrical steel sheets in which the axis of easy magnetization <100> is oriented in random directions in the plane and grain-oriented silicon steel sheets in which the axis of easy magnetization <100> is preferentially oriented parallel to the rolling direction.
However, when these steel sheets with good magnetic properties are used for, for example, a magnetic shield, it is necessary to perform steps of processing the steel sheets, joining the steel sheets by electric resistance welding or the like, and assembling the steel sheets to form a desired shape. When these steel sheets are used for a stator of a motor or a rotor, the steel sheets are punched out and a plurality of sheets are laminated for use. In such a case, steps such as a punching process and a lamination process are required. Accordingly, the use of a steel sheet as a blank is disadvantageous in that complex steps are required and that an irregular area at an electric-resistance-welding area and so on is formed, resulting in degradation of magnetic properties. In order to prevent such a problem, the use of a steel pipe as a blank has also been studied.
Steel pipes with good magnetic properties may be produced by electric resistance welding of electrical steel sheets. However, the electric resistance welding of electrical steel sheets is difficult because of a high Si content of the electrical steel sheets. Furthermore, magnetic properties at the electric-resistance-welding area are degraded. Alternatively, seamless steel pipes may be produced using electrical steel billets, but it is difficult to perform the process of pipe production because electrical steels have low ductility.
To solve the above problems, for example, Japanese Unexamined Patent Application Publication No. 2-236226 proposes a method of producing an electrical steel pipe in which a steel having high Si and Al contents is used, a seamless pipe is formed by hot extrusion and hot rolling under appropriate conditions, the seamless pipe is rolled at the recrystallization temperature or a lower temperature, and final annealing is performed. However, the technique described in Japanese Unexamined Patent Application No. 2-236226 is disadvantageous in that the hot extrusion process is essential, thereby increasing the production cost.
Japanese Examined Patent Application Publication No. 7-68579 proposes a method of producing an electrical steel pipe in which a steel slab or a cast slab having a steel composition containing 99.5% or more of iron (Fe) and the balance being impurities is heated to 1,100° C. to 1,350° C. and hot rolling is performed to prepare a blank, a pipe is then produced, and the pipe is heat-treated at 500° C. to 1,000° C. According to the technique described in Japanese Examined Patent Application Publication No. 7-68579, a steel pipe having satisfactory properties for a magnetic shield is produced. However, in this technique, grain growth is merely induced by the heat treatment, and crystal orientations are not considered. Therefore, the properties of this steel pipe are not satisfactory for applications in which excellent magnetic properties are required.
It could therefore be advantageous to provide a steel pipe with good magnetic properties that is suitable for use in a magnetic shield or a motor and a method of producing the same.